1. Field of the Invention
The present invention relates to a pulse power amplifier of the type which comprises a driver stage for receiving an input pulse to be amplified and a splitter/combiner stage following the driver stage for power division onto two output stages connected in parallel thereto.
2. Description of the Prior Art
A pulse power amplifier of the type generally set forth above is known from Application Note AV-201, June 1978, of the Microwave Semiconductor Corporation.
High-frequency power transistors are employed for pulse operation in pulse power amplifiers that generate peak pulse powers of 200W and above into the GHz range. The obtainable pulse power of individual transistors is limited and lies on the order of magnitude of 100W, dependent on the pulse format. For thermal reasons, the transistors thereby operate in the non-linear class C mode and are only driven into the class A mode or class B mode during the gating operation.
Higher output powers are achieved by parallel circuits having a plurality of transistors. The amplifier circuits thereby contain splitter/combiner stages as decoupling elements and, in order to divide or, respectively, to combine the signals to be amplified or, respectively, the amplified signals. As dividers, 3 dB power dividers, or what are referred to as 3 dB hybrids are employed as splitter/combiner stages. Such "hybrid couplers" are known, for example, from the catalog of the ANAREN Company. When the output signals of the hybrids have a relative phase .+-.90.degree. or of 0.degree. and 180.degree. relative to one another, then they are called a 9020 hybrid or a 180.degree. hybrid. Dividers such as 3 dB power dividers referred to as three-ports or four-ports that symmetrically divide power supplied at one port onto two other ports are known in the art. In four-port power dividers, one port is also decoupled. In this connection reference may be taken to R. K. Hoffmann, Integrierte Mikrowellenschaltungen, Springer Verlag, pp. 14-15. In the known amplifier circuits, this port is terminated with an absorber resistor.
In certain applications of the pulse power amplifiers, it is not only a fixed, high-output power that is required; rather, a maximum and a minimum value of the output pulse power is required in view of low detectability and different distances to be bridged. The output pulse power can thereby differ by 10-30 dB.
It is conceivable to reduce the power output by the pulse power amplifier in that the drive power of one of the two output stage transistors connected parallel to one another is disconnected on the basis of a switch-over device. What is referred to as a SPDT (single-pole, double-throw) switch can thereby be employed as a switch-over device (in this connection see Application of PIN Diodes, Application Note 922 of Hewlett Packard). The switch losses must thereby be compensated by an increased driver power. Given reduced output power, the activated output stage transistor continues to work at full gain and power, this having a disadvantageous affect on the efficiency and on the reliability of the overall circuit.